Automatic frequency control



\ H. L. DoNLEY Erm.

AUTOMATIC FREQUENCY CQNTROL May 9, 1953 Filed Feb. 25, 195o VENTO/YS L5 111.@

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Patented May 19, 1953 AUTOMATIC FREQUENCY CONTROL Hugh L. Dcnley and Eugene O. Keizer, Princeton,

N. J., assignors to Radio Corporation of America, a corporation of Delaware Application February 25, 1950, Serial No. 146,166

(Cl. Z50-20) 7 Claims.

This invention relates to automatic frequency control (AFC) devices, and more particularly to thermal AFC devices which may be used in frequency modulation (FM) radio receivers or in the audio or sound channels of 'television (TV) receivers.

The usual AFC of an oscillator by means of a reactance tube is of limited use in a FM or TV receiver because of the lack of adequate sensitivity and the loading eiect of the reactance tube on the oscillator. Thus, the requirement lof additional gain to make the reactance tube feasible as an AFC for FM or TV receiver application is practically ruled out by cost considerations alone. The limitations of the usual reactance tube circuits necessitated the development of a special tube, the transitrol, for AFC purposes. The diniculty of such inherently instantaneous AFC devices, When applied to FM receivers at least, is the possibility of channel skipping when the FM receiver is manually tuned.

The AFC action could be slowed down by means of a thermal device depending for example upon the motion of a bimetallic strip to control the oscillator frequency. In general, a device of this sort, or in fact most thermal devices, would be impractical because of the long time constant involved, poor power sensitivity and the tendency to be microphonic.

A thermal AFC arrangement according to this invention overcomes the above-described objections by providing a simple, cheap, small, sensitive device whose time constant is long enough as compared to instantaneous AFC systems to permit easy and accurate manual tuning of a FM receiver and is yet short enough as compared to other thermal AFC systems to be of no disadvantage.

According to this invention, the extremely large temperature coeiiicient of capacitance of certain compositions of barium strontium titanate over certain ranges of temperature is utilized. The titanate dielectric is maintained in intimate thermal contact with a very small heater element so that the capacitance of Ithe dielectric element rapidly varies in response to changes in temperature produced Iby changes in current flow through the heater element. When this heater current is controlled from the discrimina-tor of a FM or TV receiver, this current Will be proportional to the frequency shift of the local heterodyne oscillator. This frequency shift or drift is then corrected automatically thr-ough the change in the capacitance of the dielectric element, which capacitance is coupled to the local oscillator circuit. Since the current or control power for heating the ca'- pacitor is derived from 'the output of the discriminator, With say one stage of D. C. amplication, the heater should have a high resistance to match7 the amplifier stage, or in other words so that it can serve as the plate resistor of the amplifier stage. Although barium strontium titanates have been mentioned, other dielectrics will function in the desired manner, as long as the dielectric constants of these other dielectrics vary with temperature. Such other dielectrics ordinarily are of less sensitivity, however.

In tests conducted onthe action of such a unit as that just described (comprising a high-temperature-coeiiicient-of-capacitance titanate dielectric element in intimate thermal contact with a small high resistance heater element), when applied to a FM receiver, it was found that it would be desirable to have the thermal AFC unit suiiiciently slow-acting to permit the receiver to be manually tuned in a normal manner, after which the AFC action would correct any mistuning or subsequent oscillator drift. It was found that to accomplish this purpose (since the time constant of any thermal device depends on its thermal capacity or roughly the mass lof the device), there was required a thermal AFC unit which took approximately one minute to warm up to its operating temperature after the receiver Was turned on. In this connection, it is desired to be pointed out that the capacitor should be :operated well above room temperature, and in fact above the ambient temperature in the receiver, to render such capacitor unaifected by ambient temperature changes and to eliminate the necessity for careful thermal shielding of the capacitor. The start-up time of the receiver itself, however, from energization to operation, Was found to be only about 15-20 seconds.

VOne difficulty encountered when the warm-up time of the AFC unit exceeds the start-up time of the receiver itself is that the oscillator frequency may be at one extreme of its control characteristic (corresponding Ito a relatively low temperature of the control capacitor) when a `signal is first received, and if a signal is present at this time the resulting AFC action holds the oscillator in this undesirable position. Another manifestation of the same effect may be noticed when the FM receiver is turned on a minute or two after it has been turned off While properly tuned to one ofl two closely spaced signals. When turned on, the receiver may select the wrong signal.

One solution to the aforementained difficulty might be to delay the start-up of the receiver for a minute or so; however, this would be an undesirably long period.

, Therefore, an object of this invention is to A dielectric element having-a; high temperature .1 c

coeiiicient of capacitance is provided in thermal contact with a heater elementfthecnrrent through which is controlled by the D.' C. output of a'discriminator in a FM receiver. The capacitance of the dielectric element iswcoupled to the tank circuit of` a' local oscillator inthe rec-eivei` to control the frequency-of such oscillator. An auxiliary or additional heater, energized from .the power source, is provided yto maintain the dielectric element at its operating-temperature when the receiver is turned 01T.

Certain titanate materials, such as a .mixture of barium and strontium titanates, .possess very high dielectric-constants (on the .order .of l0,000, for example) and in additionpossessa definite critical (Curie) ktemperature Yabove vand .below which the dielectric constant rapidly decreases. The position of the vCurie temperature .on the temperature scaledepends onthe. composition .of the titanate material; y:as -the..strontium content of thebarium-strontiumtitanate composition is increased the point ofmaximum .dielectric constant moves down on the temperature scale in approximately linear fashion. For theY composition '70%.Ba'IiO3-30% -SrTiO3.the Curie tempera.- ture comes just about atroom temperature. Occasionally, it may be desirable toadd a small amount of calcium titanate Itothebarium-strontium titanate composition; this small amount of calciumv titanate .gives at .100 mc., a capacitor with somewhat `lower -lossthen the straight 'T0/30 barium-strontium titanate-composition. With thesetitanate materials, there is.an extremely rapid drop in capacitanceattemperatures.above the Curie temperature; withfsuch titanate-compositions as that .previously described, thereis an extremely high temperature coeflicient of capacitance.

Above the Curie .temperature the dielectric losses are reduced and the nonlinear properties of the dielectric disappear. On the lowtemperature side of the Curie point the dielectric'losses increase and nonlinear electrical characteristics become pronounced. Therefore, to avoid these losses and dielectric anomalies and .alsoto minimize ambient temperature effects, operating temperatures -Well yabove thefCurie point are preferable. However, at temperatures below the Curie-temperature thedielectric constant drops as the temperature changes in roughly the same fashion las for temperatures above the/Curie point, so that such-temperatures .are usable for the device.

As-a typical example, -a dielectric chosen for the capacitive element inthe thermalAFC device of this invention mayhavethe composition '70% barium titanate 30% strontium titanate yand may be operated at a-.temperature of roughly 40 C., an operating temperature Well'above ambient temperature in a FM receiver. Such afcapacitive elementis illustrated 'at Ifn'Fig. lfasbeing. a

rectangular prism of length L, width W vand height I-I. At this operating temperature of 40 C. the dielectric is very sensitive to temperature changes, as exemplified by a temperature coefcient of capacitance of some 30,000 parts per million per degree C. Consequently, the very large temperature coefficient of capacitance together'with the high dielectric constant of this material evenvvhen operated'at a temperature well above room temperature make possible a very small, very sensitive thermal element of low nthermal Vcapacity so that temperature changes can be rapi'dly'followed provided a low thermal :capacityqheating vdevice is used. Element l is l shown'gr'eatly exaggerated in Fig. 1. In a typical exampleQL-Was'- 250 mils, I-I was 130 mils and W was v40.y

In general, heating the temperature-responsive capacitor by means of a detached space type heater would result in too great a thermal lag for most circuit applications. A .metal-type heaterV soldered directly to the dielectric element meets the requirement of highv thermal conductivity between heater and capacitor `and .issatisfactory in those applications wherelow resistance is permissible. In the applications of chiefconcern here, howevensuch as the application .to a FM receiver, the massand Asize of the unit must be kept Asmall, and in such-applications: the control power to heat the -smallfcapacitor is preferably obtained from ytheplate circuit of a Vvacuum. tube. Hence, the heater should have a high resistanceto match the tube resistance. In addition, good thermal conductivity between fthe-dielectric and heater must be maintained.

In'order to satisfy the requirement-cf high resistance per unit volume, 'a `semiconductive heater elementZris used, this element being composed, for-example, of 'mafgnetite (FecOl) vand barium Astrontium ltite-nate in the correct propor- -tion to give the desired high resistance value in the chosen Asniall'volume 'of heater. The resistance fora given volume varies over wide limits in accordancewith'the magnetite tol titanate proportions. For example, the lresistivity of the heater can'be'changedr from about 32,000 ohmom. for 10% FesOi' and 90% 70/30 (Ba/Sfr) TiOa to 32 ohm-cm.'when the' magnetite percentage is increased to Consequently, with such Ia device the rcapacitance 'changes' 'rapidly with small changes in'heater powerandV yet it` ispossible to match the heater resistance 'to the plate resistance of a control' tube', for example'without increasing appreciably the massof the heater. For a. convenient size heater, such as one 40 mils in diameter and 40'mils inlength, which may be thev size indicated in Fig. 1, a heater composition of 30% FeaOi and 70% (Ba/Sr)'IiO3 gives a resistance of about 150,000v ohms at a heater ourrent of roughly l@ to l milliampere at the operating temperatureof 40 C. or vWhere the capacitance of the thermal AFC unit is one-half .its room temperature or cold value. Magnetite, a typical semiconductor, possesses a very large negative temperature coefficient .of resistance as a consequence of 'which the `heater resistance drops several fold as the current through the heater is increased. The .preparation and characteristics of such a heater are more fully described in the copending Carlson et al. application, Serial No. 704,016, filed October 18, 1946, now Patent-#2,528,113, dated October 3l, 1950.

It'has been found that a semiconductive heater may also be ymade by introducing vthe correct amount (for: a :desired rres'stant) :offc'opperttany. ate to iron oxide (FecOs) and then firing at a high temperature. A 60% CuTiOa 40%v FeaOa composition has been found satisfactory for heaters. Other compositions could also be used for heaters. n

As previously stated, heater element 2 may be of cylindrical configuration, and may for example have a diameter X of 40A mils (equal in this example to the dimension W) and a length X of 4'0 mils. Each circular end face of element 2 is equipped with a fired-on silver electrode, obtained by firing at '750-800F C. a silver paste applied to such faces. 'I'hese electrodes cover such end faces, the upper lend electrode being indicated at 3 and the lower end electrode atv 4. The capacitive or dielectric element I is also equipped on its bottom face with a fired-on silver electrode aligned with electrode 3. In order to ,fasten elements I and 2 together and to provide the intimate thermal contact desired, the capacitive element I and the main heater element 2 are then sweated together with a small amount of soft solder, the heater element 2 thus being secured to the lower of the two faces defined by dimensions L and W, at one end of such face, in intimate thermal contact with the capacitive element I. Since the diameter X of theI heater element is equal tol the width W of the capacitive element, the heater elements circular end face extends from side to side of the capacitive element. In order to provide electrical connections to electrodes 3 and 4, short lengths of copper leads 1 to 3 mils in diameter (not shown in Fig. 1) are soldered toi such electrodes to provide supporting connections of low thermal but high electrical conductivity.

As previously stated, the time constant of any thermal device depends on its thermal capacity or roughly the mass of the device.' For use in a FM receiver, it is desirable to have the thermal AFC unit sufficiently slow-acting to permit the receiver to be manually tuned in a normal manner, after which the AFC action would correct any mistuning or subsequent oscillator drift. The dimensions of the dielectric element and heater element previously stated in connection with Fig. 1 were found to give about the right mass of unit to provide a control time (the time required for the AFC to retune the receiver when the tuning of the receiver slowly drifted) of roughly -15 seconds. However, it was found that a dielectric element of this size had a larger capacitance than was needed for the control of the particular oscillator with which the element was to be used; only a fraction of this capacitance was desired to be coupled to the oscillator. Therefore, a fired-on silver electrode 5, similar to electrodes 3 and 4, is applied to only a portion of the upper face of element I, electrode 5 extending over the full width W of the element but having a length X small in comparison to length L of element I, length X being 40 mils, for example. A dielectric element I having the dimensions stated above has a warm-up time (the time required to reach the operating temperature of about 40 C.) of 1-11/2 minutes.

Now referring to Fig. 2, which shows in part schematic and part detailed form a circuit dia-` gram of a FM reeciver utilizing this invention, a power supply 6, energized from the power line through a series main power switch 1, supplies plate voltage to all tubes of the receiver, the

plate voltage lead being labeled B+. The heaters of all tubes are energized whenswitch I is closed. The receiver includes in cascade, in the signal 6 channel, RF amplifier 8, a converter stage 9 to which is supplied heterodyning energy from a local 'oscillator I0, IF amplifier II and a dis-y criminator or ratio detector I2 which has a pair of output leads I3 Yand I4, lead I4 being grounded. When the receiver is in operation, the discriminator I2 will, at any instant of time, supply voltages across leads I3 and I4 which include a D. C. component. This component, unless it is of zero magnitude, will have some iinite positive or negative value. The magnitude and polarity of ythis component are responsive to slow changes in frequency, from a predetermined value, of the mean signal frequency fed to discriminator I2.

Across leads I3 and I4 will also apear, as is well-known to those skilled in the art, audio frequency modulating voltages. 'I'he audio frequency voltages are coupled over condenser I5, volume control potentiometer. I6 and condenser II onto the grid I 8 of the rst audio stage amplifier tube ISI the cathode 20 of which is grounded through resistor 2|. The D. C. component of the discriminatory output voltage is coupled through resistors 22 and 23 onto control grid I8 of tube I9, which is thus used as a control or D. C. amplifier tube, the resistors 22 and 23 being connected in series between output lead I3 and grid I8; a radio frequency (RF) bypass condenser 24 is connected from ground to a point between resistors 22 and 23. Plate 25 of tube I9 is connected to main heater electrode 4 of heater 2, while heater electrode 3 is connected to the B supply; vin this Way, the plate load resistance or output resistance of tube I9 is the main heater 2 of the AFC unit. A RF bypass capacitor 25 is connected from ground to plate 25, while the amplied audio frequency voltage is coupled through condenser 21 to the grid of the following audio stage.

Fixed bias is applied to the cathode 20 by any suitable arrangement, for example by means of a vbleeder resistor 28 from the positive supply lead in combination with resistor 2I, a RF bypass condenser 29 being connected from such positive lead to ground. Resistor 28 is used to set the initial or operating bias on tube I9 such that the initial plate current through the main heater 2 (when the receiver is energized) maintains the operating temperature of the capacitor I (for example, 40 C.) well above ambient temperature when there is a zero or correct tuning point control voltage out of the discriminator, thereby eliminating the necessity for careful thermal shielding of the capacitive element. Although 40 C. has been mentioned as the operating temperature, it is to be understood that in some cases (for example, in TV receivers wherein the ambient temperature in the receiver is generally y higher than in FM receivers) the operating temperature ofthe unit may be much higher, for example C.

The power requirement of the heater 2 is small, and, as previously stated, the resistivity of the heater material may be so chosen -that the function of D. C. amplification may be accomplished in one of the existing tubes in the receiver. For example, heaters with resistances on the order of 100,000 ohms operating at about 0.5 to 1.0 milliampere are-well suited for use in place of the plate load resistors of commonly-used highmu triode first audio amplifier tubes. Other possibilities with different resistances for the heater may be as screen or cathode resistors for control tubes.

Although it has beenstated that theheater elementmay be usedtas the plate load resistor. or screen resistor orv cathode. resistor. of` control case ifqtheheater were. connected elsewhere. inv

the` controltube circuit. Forexample, if the heaterwere. placed `in the cathode. circuit-of the tube,l a .loss in sensitivity of .control` occurs vwhen a plate resistor is used since the heater and plate resistor are then in series. Also, by utilizing. the heater element as output load-.or plate load for they control ltube, thegainoi such tube as an amplier is not impaired.,

The capacitor element `I is connected into the tank circuitiof localcscillator IIJ of the.- superheterodyne FM receiver. by a connection which extends. from.. capacitor electrode through aseries. capacitor 39'. to the-plate of thev tube in oscillator IIJ. In this. way, the capacitive. element- I.is made .part offthe oscillator circuit of the.=.rec.eiver. Byvirtue of the-RF bypass. condenser 29. connectedztoN electrode 3, the capacitance of the dielectric element I between the-opposite electrodes 3 and appears in effect betweenelectrode 5 and ground.

We Vwill assume. that when the receiver is rst energized-by closing switch 1, the capacitor I is substantially at its. operating temperature. This is a valid assumption, asl will hereinafter appear. The titanate capacitor-heater unit I, 2 is then ready to function as a frequency control for the oscillator when theV receiver is first operative. The tube. capacitors and circuit constants of the oscillator IB will continue to change. forl some time after thereceiver is turned on, thereby to cause rthe'D.y C. component of the discriminator output voltage to change from azero value. If

the discriminator output voltage polarity with regard to frequency changeat its input has been correctly predetermined; this change in discriminator output voltagev will cause the. temperature of the titanate capacitor I to change in the proper direction to-counteractthechange in other circuit coinponents'o vthe oscillator I0. For example, if the discriminator polarity issuch that an increase inthe meanf intermediate frequency applied thereto appliesanegative voltage to grid I8 to reduce the heater- 2f current, the temperature of-the capacitor- I will decrease and thev termediate frequency` In those receivers where.. the. locall oscillator frequency is'lower than the.

signal frequency the polarity of the discriminator should-fbe reversed from that given above.

Because of the control time or timeconstant of the Fig. l unit (l0-15 seconds) the manual tuning process is similar to that in a receiver with no AFC and hence there is little tendency to` hold on a strong station and jump over an adjacent weak one. Thus, the operator can immediately tune in a station and the AFC action will compensate for tendencies of the oscillator to drift. If when the receiver is. firstoperative the .tuning point isnearly correct'vfor a particular station, the AFC action will correct the tuning of the Voscillator in the proper direction to tunev to the station.

Good over-alloperationhas been experienced where the coupling ofthe capacitive section I of the unit to the local oscillator I0 is such that about a ytwo-to-one change in capacitance of the unit is required for the desired range of control.-

Seriescapacitor 3B between the oscillator IIB and the capacitive section I of the AFC unit is a convenient way to reduce the coupling to the oscillator circuit. Thedesiredcoupling can be calculated from a consideration of the circuit capacitance and the AFC-unit capacitance values.

The above discussion has proceeded on the assumption that when the receiver is first turned` start-up time ofthe receiver itself v (which is the caseinthe Fig. l unit, where the warm-up time.

of the capacitor l is 1 11/2 minutes and the startup time of the receiver is 15-20 seconds), certain diiiiculties ensue. One difficulty is that the oscillator frequency'may bev at one extreme of its control characteristic (corresponding to a relatively low temperatureof the control capacitor) when a signal is first received, and if a signal is present the resulting AFC action holds the oscillator in this vundesirable position. Another difliculty is that, when the receiver is turned on after it has been turned o while tuned to one of two closely spaced'signals, it may select the wrong signal.-

To delay start-up of the receiver would be iinpractical,

According to this-invention, an auxiliary heater is provided on the AFC unit, which heater maintains the capacitive element essentiallyA at4 its operating temperature while the receiver is deenergized, so that the AFC unit` assumes control within lseconds after the receiver is turned on (i.. e., the receiver` start-up time). Referring again to Fig. l, they auxiliary heater 3i is preferably ofthe same size and conguration as the mainheater 2'previously. described, and is preferably similar to such main heater in all respects, having electrodes 32 and-33 similar to electrodes Sand d on its upper and lower circular end faces, respectively. Auxiliary heater 3| is secured to dielectric block I in intimate thermal engagement therewith, preferably on` the end of thev blocks bottom face opposite to .main heater 2, as illustrated.

In order to energize-theauxiliary heater 3| in such a way thatthe-heater draws a small amount iofcurrentfrom the 11G-volt A. C. power line when the receiver power is 0E, thereby to maintain element I at its operating temperature while thereceivei is ofi, a lead extends fromone side of power switch 'I to auxiliary heater electrode 33, and a lead extends from the other side-of switch 1 (througha series resistor :i4-if desired,

to applyV only anadjustable portionof the Since by the use of this auxiliary heater the operating temperature of the dielectric elementl is maintained when the receiver is turned off, the AFC unit is up to its operating temperature and ready to function as soon as thereceiver itself starts up; therefore, by using this scheme the normal receiver start-up time of about 15 seconds can be maintained, even though the dielectric element has a Warm-up time of 1-112 minutes.

Tests with a unit of the type illustrated in Fig. l installed in a FM receiver showed that the station to which the receiver was tuned when it was turned off would invariably also be the one automaticallyselected when the receiver was turned on, even if there were another closely spaced signal. Also, with this construction the AFC action holds the oscillator substantially in the mid-portion of its control characteristic.

It is desired to be made clear at this point that, although the auxiliary heater 3l is energized during the entire time that the receiver is off, the power consumed thereby is inconsequential, since such heater draws on the order of only 20 milliwatts of power. lAlso, during the operation of the receiver, only this very small power is'necessary to heat element I for AFC purposes. Although there is a closed series circuit from the power line to power supply 6 through auxiliary heater 3l when switch 1 is open, this is a very high resistance circuit and the amount of current flowing therethrough -(e. g., 0.2 ma. as compared to at least 500 ma. when switch 'I is closed to energize the receiver) is entirely insuicient to produce any energization of the rectifiers in power supply 6 at this time.

It has been found that the eifect of variations in line voltage on the AFC action is very small. In a test, using Ia Variac to control the line voltage, it was found that neither slow nor rapid variations from 80 to 130 volts caused a noticeable effect in the output signal except for a slow change of volume due to the change of gain in the FM receiver.

The small change in plate current (in tube I9) necessary to vary the capacitance of the titanate capacitor in accordance with the D. C. component of the discriminator output voltage does not change the audio gain of this stage appreciably, so that the audio stage tube may also be used as an AFC control tube. In a test of a device constructed according to this invention, in a FM receiver manufactured by applicants assignee, the local oscillator frequency was changed approximately 350 kc. per volt of change at the discriminator or ratio detector output. The control range of the receiver was about i200 to i-500 kc. for normal signal strength encountered in the Princeton area, say 100 microvolts.

Although the emphasis in the foregoing has been on a circuit application of a thermal capacitor for AFC purposes, other circuit applications are possible. For example, it is possible to change the oscillator tuning by the simple expedient of varying the initial bias on the control tube I9. In addition to the oscillator tuning, AFC action about any given bias or capacitance value is also possible when the discriminator I2 is employed. By adding another capacitance section to the thermal capacitor the RF stage .8 as well as the local oscillator I could be tuned by the change in bias of the control stage. For the FM band, for example,the required tuning range is possible with only one lto two volts change in bias on a type 6AT6 tube as the confit) trol stage tube I9. It hasbeen found possible to readily obtain better than a ten-to-one change in capacitance for 0.6 watt heater power in a fairly massive unit It maybe seen that remote tuning of oscillator IIJ can be accomplished merely by changing the bias on control stage I9.

What we claim to be our invention is:

1. In a radio receiver adapted to be energized at will from a source of power through a switch, signal-responsive means operative in response to closing of Asaid switch to produce an output voltage in response to a slow change, from a predetermined '.frequency, in the frequency of the signal fed thereto, a controllable-frequency generator in said receiver for varying the frequency of the signal fed to said means, a capacitive element the capacitance of which varies with the temperature thereof, controllable means for heating said element, means responsive to said produced voltage for controlling the heating effect of said heating means, means coupling said element to said generator, the `arrangement being such that the frequency of said generator is controlled, thereby varying the frequency of the signal fed to said signal-responsive means, in response to variations in capacitance of said element, and means energized in response to deenergization of said receiver for heating said element, thereby to maintain said element substantially at a predetermined temperature during periods of deenergization of said receiver.

2. A radio receiver as dened in claim l, wherein the last-mentioned means is an electrical heater element in intimate thermal contact with the capacitive element and wherein such heater element is connected across the said switch.

3. In a radio receiver adapted to be energized at will from a source of power through a switch, signal-responsive means operative in response to closing of said switch to produce an output voltage in response to a slow change, from a predetermined frequency, in the frequency of the signal fed thereto, a controllable-frequency generator in said receiver for varying the frequency of the signal fed to said means, a capacitive element the capacitance of which varies with the temperature thereof, an electrical heater element in intimate thermal contact with said capacitive element, means responsive to said produced voltage for controlling the current through said heater element, means coupling said capacitive element to said generator, the arrangement being such that the frequency of said generator is controlled, thereby varying the frequency of the signal fed to said signal--responsive means, in response to variations in capacitance of said capacitive element, and means energized in response to denergization of said receiver for heating said capacitive element, thereby to maintain such element substantially at a predetermined temperature during periods of deenergization of said receiver.

4. A radio receiver as defined in claim 3, wherein the last-mentioned means is another electrical heater element in intimate thermal contact with the capacitive element and wherein such other heater element is connected across the said switch.

5. In a tunable receiver adapted to be energized at will and having a predetermined startup time, a temperature-responsive variable capacitor coupled to said receiver to control the tuning of one -stage thereof, controllable means operative in response to energization of said receiver for heating said capacitor to a predeterfininedv 'operating temperature, "capacitor i havingA a Warm-up time, from room temperature to said operating temperature; which exceeds the receiver start-up time, a discriminator in said receiver having its input coupled to said lone rev ceiver stage, means responsiveto the output of said discriminator for controilingsaidv heating means to vary the temperature of said capacitor with respect to said operating temperature,l and ymeans for maintaining said capacitor substantially at said operating temperature 'during periods of deenerg'ization of saidV receiver,

6.'In ar tunable 'receiver adapted to *be enerv'giz'ed"at"vv11 froma source of power through va switch and having a predetermined start-'uptime, a' temperatureresponsive variable capacitor coupled to said receiver to"'control thetun j ing of one stage thereof, an electrical heater-ele- "ment in intimate thermal* contact With {saidV capacitor, said element'beingfoperative in response to energiza'tion of sai-d receiverforjheatingsaid f capacitor to a p redetermined vroperating temperature, said capacitorhavingla warmt-iup time, from room "temperatureto said operating temperature,

Which exceeds the receiverrstart-iup timefadiscriminatorinlsaid receiver 'havingjits' yinput coupied to said one receiverv stagefmeans responsive tothe output of saidvv discriminator for varying operating temperature, another'V electrical heater i element in intimate thermal "contact Awith said capacitor, and (means connecting vsaid v'otner heater element across' said switch, therebv to `mined temperature during periods of d'eenergimaintain said capacitor substantially at'said operating temperature during periods of 'deenergipled' to the output of said discrimina'tor and having `a plate circuit, means coupling said heater element to`said plate circuit as the 'piate'load 'resister for said tube, to thereby control thetemperature of said capacitor, when said receiveris energized,A in dependence upon the direct'voltage output' of said discriminator, and means energized in response to deenerg'ization of said regceiver for heating said capacitor, therehy'to maintain the same substantially at a predeter- Zation of said receiver.

HUGH L. DONLEY. EUGENE O. KEIZER.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date i 1,894,687 Hyland Jan. 17, 1933 2,210,41L06 Henderson Aug. 6, 1940 2,399,082 Wainer Apr. 23, 1946 2,483,070 Spindler Sept. 27, 1949 

